I couldn't really answer your question myself, as I'm only on day 16 of my first, ever, incubation 'journey'; however, I found the following information at
http://chickscope.beckman.uiuc.edu/resources/egg_to_chick/procedures.html and I highlighted the section, in red font, that I found most applicable...
HOW THE CHICK EMERGES FROM THE SHELL
The head of the chick develops at the large end of the egg. Between the 15th and 16th days, the chick orients itself so that its head is near the air cell at the large end of the egg. Not long before the chick is ready to attempt to make its way out of the shell its neck acquires a double bend so that its beak is under its right wing and pointed toward the air cell. About the 19th day the chick thrusts its head forward. Its beak quickly breaks through the inner shell membrane, and the chick's lungs begin to function. Complete breathing by the lungs usually does not occur until the 20th day of incubation.
Using its egg tooth (a tiny, sharp, horny projection on the end of its beak), the chick pecks at the shell thousands of times. Finally, the young bird pips its way through the shell and begins to breathe air directly from the outside.
After the chick has made a hole in the shell, it stops pipping for three to eight hours and rests. During this time, it is acclimating its lungs to the outside atmosphere. After the resting stage is completed, the second stage of pipping begins.
The chick begins to turn slowly inside the egg. As it turns, usually counter-clockwise, the cutting edge of the chick tooth continues to chip away. In two to five hours, the chick has made about three quarters of a turn inside the egg. As the chick progresses in its movement around the shell, it begins pushing on the egg cap (large end). Squirming and struggling, the chick works feverishly for about 40 minutes pushing at the cap. Finally with a vigorous shove, the chick breaks free from the shell, still wet and panting.
When the chick is freed completely from the shell, it lies still. Its energy has been virtually exhausted, and it is extremely tired. After a rest of some few minutes, the chick begins to rise to its feet and gain coordination of its muscles. Within a few days the egg tooth, its usefulness over, will disappear.
Hope that helps!
